Solid state lighting apparatus and methods using integrated driver circuitry

ABSTRACT

A lighting apparatus includes a first substrate including a switching circuit, the switching circuit including a first port, a second port and a current control circuit configured to generate a current at the second port of the current control circuit responsive to a varying voltage at the first port. The apparatus further includes a second substrate mounted on the first substrate and including at least two LEDs electrically coupled to the second port of the current control circuit of the first substrate.

CROSS-REFERENCE TO RELATED PATENT APPLICATIONS

The present application is a continuation of U.S. patent application Ser. No. 13/192,755, filed Jul. 28, 2011 entitled SOLID STATE LIGHTING APPARATUS AND METHODS USING INTEGRATED DRIVER CIRCUITRY, the disclosure of which is hereby incorporated by reference in its entirety.

FIELD

The present inventive subject matter relates to lighting apparatus and methods and, more particularly, to solid state lighting apparatus and methods.

BACKGROUND

Solid state lighting arrays are used for a number of lighting applications. For example, solid state lighting panels including arrays of solid state light emitting devices have been used as direct illumination sources, for example, in architectural and/or accent lighting. A solid state light emitting device may include, for example, a packaged light emitting device including one or more light emitting diodes (LEDs), which may include inorganic LEDs, which may include semiconductor layers forming p-n junctions and/or organic LEDs (OLEDs), which may include organic light emission layers.

Solid state lighting arrays are used for a number of lighting applications. For example, solid state lighting panels including arrays of solid state light emitting devices have been used as direct illumination sources, for example, in architectural and/or accent lighting. A solid state light emitting device may include, for example, a packaged light emitting device including one or more light emitting diodes (LEDs). Inorganic LEDs typically include semiconductor layers forming p-n junctions. Organic LEDs (OLEDs), which include organic light emission layers, are another type of solid state light emitting device. Typically, a solid state light emitting device generates light through the recombination of electronic carriers, i.e. electrons and holes, in a light emitting layer or region.

Solid state lighting panels are commonly used as backlights for small liquid crystal display (LCD) screens, such as LCD display screens used in portable electronic devices. In addition, there has been increased interest in the use of solid state lighting panels as backlights for larger displays, such as LCD television displays.

Although solid state light sources having high coloring rendering index (CRI) and/or high efficiency have been demonstrated, one problem with the large-scale adoption of such light sources in architectural applications is that commercial lighting systems utilize lamps with standardized connectors that are designed to be used with alternating current (AC) power, which may be phase cut using a phase cutting dimmer device. Typically, a solid state lighting source is provided or coupled with a power converter that converts AC power into DC power, and the DC power is used to energize the light source. However, the use of such power converters may increase the cost of the lighting source and/or the overall installation, and may reduce efficiency.

Some attempts at providing solid state lighting sources have involved driving an LED or string or group of LEDs using a rectified AC waveform. However, because the LEDs require a minimum forward voltage to turn on, the LEDs may turn on for only a part of the rectified AC waveform, which may result in visible flickering, may undesirably lower the power factor of the system, and/or may increase resistive loss in the system.

Other attempts at providing AC-driven solid state lighting sources have involved placing LEDs in an anti-parallel configuration, so that half of the LEDs are driven on each half-cycle of an AC waveform. However, this approach requires twice as many LEDs to produce the same luminous flux as using a rectified AC signal.

SUMMARY

Some embodiments provide a lighting apparatus including a first substrate including a switching circuit, the switching circuit including a first port, a second port and a current control circuit configured to generate a current at the second port of the current control circuit responsive to a varying voltage at the first port. The apparatus further includes a second substrate mounted on the first substrate and including at least two LEDs electrically coupled to the second port of the current control circuit of the first substrate.

In some embodiments, the at least two LEDs may include first and second end nodes coupled to the second port and the switching circuit may include respective terminals coupled to respective intermediate nodes of the at least two LEDs. The switching circuit may be configured to selectively bypass at least one LED. The switching circuit may be configured to vary a number of conducting LEDs responsive to the varying voltage. For example, the switching circuit may be configured to increase the number of conducting LEDs responsive to an increasing magnitude of the varying voltage.

In some embodiments, the apparatus may further include a rectifier circuit coupled to the first port and the switching circuit may include a buffer circuit coupled to an output of the rectifier circuit, a resistor ladder circuit coupled to an output of the buffer circuit and a plurality of switches, respective ones of which have control inputs coupled to respective nodes of the resistor ladder circuit. The apparatus may further include a voltage-controlled current control circuit electrically coupled to the output of the rectifier circuit and to the second port.

In some embodiments, the second substrate may be flip chip bonded to the first substrate. The first substrate may include a silicon substrate, a gallium nitride substrate, a silicon carbide substrate or a graphene substrate. The at least two LEDs may be arrayed on a surface of the second substrate opposite the first substrate. In further embodiments, the switching circuit may be configured to change between parallel and serial connections of first and second LEDs responsive to the varying voltage.

Some embodiments provide a device including a submount configured to support an LED substrate, a switching circuit integrated in the submount and including a first port, a second port and a current control circuit configured to generate a current at the second port of the current control circuit responsive to an varying voltage at the first port and a plurality of contacts on the submount, coupled to the second port of the switching circuit and configured to mate with corresponding contacts of the LED substrate. The plurality of contacts may include contacts configured couple the second port of the current control circuit to at least one of the LEDs on the LED substrate and contacts configured to couple respective terminals of the switching circuit to at least one other of contact of the LEDs. The device may further include a voltage-controlled current control circuit electrically coupled to the output of the rectifier circuit and to the second port.

The submount may be configured to support flip chip bonding of the LED substrate thereto. The submount may include a silicon substrate, a gallium nitride substrate, a silicon carbide substrate or a graphene substrate.

In some embodiments, the switching circuit may be configured to vary a number of conducting LEDs responsive to the varying voltage. In further embodiments, the switching circuit may be configured to change between parallel and serial connections of first and second LEDs responsive to the varying voltage.

Further embodiments provide a lighting apparatus including at least two LEDs and a switching circuit integrated on a common substrate, the switching circuit including a first port configured to be coupled to an AC power source, a second port coupled to first and second end nodes of the at least two LEDs, a current control circuit configured to generate a current at the second port responsive to an varying voltage at the first port and a LED switching circuit coupled to intermediate nodes of the LED string and configured to selectively bypass LEDs in the string of LEDs.

In some embodiments, the switching circuit may be configured to vary a number of conducting LEDs of the at least two LEDs responsive to the varying voltage. In some embodiments, the switching circuit may be configured to change between parallel and serial connections of first and second LEDs responsive to the varying voltage.

In additional embodiments, a lighting apparatus includes a plurality of LEDs and a switching circuit coupled to the plurality of LEDs and to a power source having a varying voltage and configured to change between parallel and serial connections among the LEDs responsive to the varying voltage.

BRIEF DESCRIPTION OF THE DRAWINGS

The accompanying drawings, which are included to provide a further understanding of the inventive subject matter and are incorporated in and constitute a part of this application, illustrate certain embodiment(s) of the inventive subject matter. In the drawings:

FIG. 1 illustrates a lighting apparatus according to some embodiments;

FIG. 2 illustrates drive circuitry for a lighting apparatus according to further embodiments;

FIG. 3 illustrates a lighting apparatus with integrated drive circuitry according to some embodiments;

FIG. 4 illustrates a lighting apparatus with integrated drive circuitry according to further embodiments;

FIGS. 5A and 5B illustrate a lighting apparatus with a submount including integrated drive circuitry according to further embodiments;

FIG. 6 illustrates a lamp device incorporating an LED with integrated drive circuitry according to further embodiments; and

FIGS. 7-10 illustrate drive circuitry and operations thereof for a lighting apparatus according to still further embodiments.

DETAILED DESCRIPTION

Embodiments of the present inventive subject matter now will be described more fully hereinafter with reference to the accompanying drawings, in which embodiments of the inventive subject matter are shown. This inventive subject matter may, however, be embodied in many different forms and should not be construed as limited to the embodiments set forth herein. Rather, these embodiments are provided so that this disclosure will be thorough and complete, and will fully convey the scope of the inventive subject matter to those skilled in the art. Like numbers refer to like elements throughout.

Embodiments of the present inventive subject matter now will be described more fully hereinafter with reference to the accompanying drawings, in which embodiments of the present inventive subject matter are shown. This present inventive subject matter may, however, be embodied in many different forms and should not be construed as limited to the embodiments set forth herein. Rather, these embodiments are provided so that this disclosure will be thorough and complete, and will fully convey the scope of the present inventive subject matter to those skilled in the art. Like numbers refer to like elements throughout.

It will be understood that, although the terms first, second, etc. may be used herein to describe various elements, these elements should not be limited by these terms. These terms are only used to distinguish one element from another. For example, a first element could be termed a second element, and, similarly, a second element could be termed a first element, without departing from the scope of the present inventive subject matter. As used herein, the term “and/or” includes any and all combinations of one or more of the associated listed items.

It will be understood that when an element is referred to as being “connected” or “coupled” to another element, it can be directly connected or coupled to the other element or intervening elements may be present. In contrast, when an element is referred to as being “directly connected” or “directly coupled” to another element, there are no intervening elements present.

It will be understood that when an element or layer is referred to as being “on” another element or layer, the element or layer can be directly on another element or layer or intervening elements or layers may also be present. In contrast, when an element is referred to as being “directly on” another element or layer, there are no intervening elements or layers present. As used herein, the term “and/or” includes any and all combinations of one or more of the associated listed items.

Spatially relative terms, such as “below”, “beneath”, “lower”, “above”, “upper”, and the like, may be used herein for ease of description to describe one element or feature's relationship to another element(s) or feature(s) as illustrated in the figures. It will be understood that the spatially relative terms are intended to encompass different orientations of the device in use or operation, in addition to the orientation depicted in the figures. Throughout the specification, like reference numerals in the drawings denote like elements.

Embodiments of the inventive subject matter are described herein with reference to plan and perspective illustrations that are schematic illustrations of idealized embodiments of the inventive subject matter. As such, variations from the shapes of the illustrations as a result, for example, of manufacturing techniques and/or tolerances, are to be expected. Thus, the inventive subject matter should not be construed as limited to the particular shapes of objects illustrated herein, but should include deviations in shapes that result, for example, from manufacturing. Thus, the objects illustrated in the figures are schematic in nature and their shapes are not intended to illustrate the actual shape of a region of a device and are not intended to limit the scope of the inventive subject matter.

The terminology used herein is for the purpose of describing particular embodiments only and is not intended to be limiting of the present inventive subject matter. As used herein, the singular forms “a”, “an” and “the” are intended to include the plural forms as well, unless the context clearly indicates otherwise. It will be further understood that the terms “comprises” “comprising,” “includes” and/or “including” when used herein, specify the presence of stated features, integers, steps, operations, elements, and/or components, but do not preclude the presence or addition of one or more other features, integers, steps, operations, elements, components, and/or groups thereof.

Unless otherwise defined, all terms (including technical and scientific terms) used herein have the same meaning as commonly understood by one of ordinary skill in the art to which this present inventive subject matter belongs. It will be further understood that terms used herein should be interpreted as having a meaning that is consistent with their meaning in the context of this specification and the relevant art and will not be interpreted in an idealized or overly formal sense unless expressly so defined herein. The term “plurality” is used herein to refer to two or more of the referenced item.

The expression “lighting apparatus”, as used herein, is not limited, except that it indicates that the device is capable of emitting light. That is, a lighting apparatus can be a device which illuminates an area or volume, e.g., a structure, a swimming pool or spa, a room, a warehouse, an indicator, a road, a parking lot, a vehicle, signage, e.g., road signs, a billboard, a ship, a toy, a mirror, a vessel, an electronic device, a boat, an aircraft, a stadium, a computer, a remote audio device, a remote video device, a cell phone, a tree, a window, an LCD display, a cave, a tunnel, a yard, a lamppost, or a device or array of devices that illuminate an enclosure, or a device that is used for edge or back-lighting (e.g., back light poster, signage, LCD displays), bulb replacements (e.g., for replacing AC incandescent lights, low voltage lights, fluorescent lights, etc.), lights used for outdoor lighting, lights used for security lighting, lights used for exterior residential lighting (wall mounts, post/column mounts), ceiling fixtures/wall sconces, under cabinet lighting, lamps (floor and/or table and/or desk), landscape lighting, track lighting, task lighting, specialty lighting, ceiling fan lighting, archival/art display lighting, high vibration/impact lighting, work lights, etc., mirrors/vanity lighting, or any other light emitting device.

The present inventive subject matter further relates to an illuminated enclosure (the volume of which can be illuminated uniformly or non-uniformly), comprising an enclosed space and at least one lighting apparatus according to the present inventive subject matter, wherein the lighting apparatus illuminates at least a portion of the enclosed space (uniformly or non-uniformly).

FIG. 1 illustrates a lighting apparatus 100 according to some embodiments. The apparatus 100 comprises a rectifier circuit 110, a voltage controlled current control (VCCC) circuit 120, a shunt circuit 130 and a string 140 of serially-connected light emitting diodes (LEDs), all of which are integrated on a common substrate 101. In embodiments described below, the common substrate 101 may comprise a variety of different types of structures, such as a printed circuit board (PCB) or a semiconductor substrate.

The rectifier circuit 110 is configured to generate a DC voltage from a power source coupled to an input 102. The power source may be an AC source, such as a utility source, or other source that generates a time-varying voltage, The VCCC circuit 120 is coupled to first and second end terminals 141, 142 of the LED string 140, and generates a current through the string 140 that is dependent upon the DC voltage generated by the rectifier circuit 110. For example, a phase cut signal, such as might be supplied by dimmer circuits commonly used in lighting applications, may be provided to the rectifier circuit 110. As the amount of phase cut varies, the VCCC circuit 120 will produce a correspondingly varying current through the LED string 140, thus causing the string 140 to brighten/darken in response to the phase cut signal. The VCCC circuit 120 may take any of a number of forms and may operate as a current source and/or a current sink for the string 140 of LEDs.

The shunt circuit 130 is connected to plural intermediate nodes of the LED string 140. The shunt circuit 130 may be used, for example, to provide power factor compensation by selectively bypassing LEDs in the string 140 responsive to the rectified voltage produced by the rectifier circuit 110. The shunt circuit 130 may use, for example, techniques described in commonly assigned U.S. patent application Ser. No. 12/775,842, entitled “AC Driver Solid State Lighting Apparatus with LED String Including Switched Segments,” filed May 7, 2010 and incorporated herein by reference in its entirety. It will be appreciated that, although FIG. 2 illustrates a shunt-type LED circuit, other types of switching circuits for LED illumination control may be integrated with one or more LEDs on common substrate as illustrated in FIG. 1.

FIG. 2 illustrates an exemplary circuit implementation of switching circuitry for use with an LED string 240 including serially connected LEDs D7-D14. The switching circuitry includes a rectifier circuit 210, a VCCC circuit 220 and a shunt circuit 230. As shown, the rectifier circuit 210 is a full-bridge diode rectifier including diodes D1-D4. The VCCC circuit 220 includes a bipolar transistor Q1 with a biasing network comprising resistors R1-R5 and diodes D5 and D6. The shunt circuit 230 comprises a plurality of switches S1-S7 that are coupled to respective nodes of the LED string 240. Control inputs of respective ones of the switches S1-S7 are coupled to a resistor ladder network comprising resistors R6-R13, which is driven by an amplifier U1 having an input coupled to the output of the rectifier circuit 210.

As will be appreciated, if the input voltage is a sinusoidal AC voltage, the output of the rectifier circuit 210 may be a full-wave rectified voltage having a waveform that includes a series of half-sinusoids repeating at twice the frequency of the input voltage. For the circuitry shown, as the magnitude of the input voltage increases in either the negative or positive voltage phase, switches S1-S7 successively open, causing the number of the LEDs D7-D14 coupled in series between the output of the VCCC circuit 220 and ground to incrementally increase as the voltage magnitude increases. This may constrain the current passing through the active LEDs to substantially conform to the input voltage waveform and, thus, may improve power factor.

It will appreciated that the circuitry shown n FIG. 2 is provided for purposes of illustration, and that other circuit implementations may be used in other embodiments. For example, the selective bypass functionality provided by the shunt circuit 230 may be provided using a variety of different circuits. For example, rather than selectively coupling nodes of the LED string 240 to ground as shown in FIG. 2, switches or other devices may be used to bypass current around selective ones of the LEDs D1-D7 using selective bypass circuitry along the lines described in U.S. patent application Ser. No. 12/704,730, entitled “Solid State Lighting Apparatus with Compensation Bypass Circuits and Methods of Operation Thereof,” filed Feb. 12, 2010 and incorporated herein by reference in its entirety. Rather than using a resistor ladder as shown in FIG. 2, a shunt circuit may use a microprocessor or other digital circuitry to control bypass switching elements for an LED string. For example, a shift register clocked by a voltage comparator circuit may be used to drive a bank of bypass switches.

Circuitry along the lines of that shown in FIGS. 1 and 2 may also be configured to provide reduced perceptible flicker that may arise from dimming using a phase cut signal. For example, in some embodiments, a VCCC, such as the VCCC circuit 220 of FIG. 2, may be modified to provide a brief “off period” during each “on” portion of a phase cut waveform, thus providing a apparent higher frequency of operation of the LED string. The LEDs in the string may be disabled when the amount of phase cut reaches the point that there is insufficient “on” time to prevent perceptible flicker.

FIG. 3 illustrates an exemplary implementation of a lighting apparatus 300 according to some embodiments. The apparatus 300 may have a circuit configuration along the lines discussed above with reference to FIGS. 1 and 2, i.e., the apparatus 300 includes a rectifier circuit 310, a VCCC circuit 320 and shunt switches 330 supported by a PCB 301. The rectifier circuit 310 and/or the VCCC circuit 320 may comprise discrete components (e.g., discrete diodes, transistors and resistors) or may be implemented as integrated circuits configured for mounting on the PCB 301. An array of LEDs 340 are also mounted on the PCB 301, and connected to the VCCC circuit 320 and the shunt switches 330. The shunt switches 330 may be controlled by a microprocessor 350, also mounted on the PCB 301. The microprocessor 350 may be used for other functions, such as communications and control.

FIG. 4 illustrates an alternative configuration for a lighting apparatus 400 according to further embodiments. The apparatus 400 includes an application specific integrated circuit (ASIC) 410 in which at least some components of a rectifier circuit 412, a VCCC circuit 414 and a shunt circuit 416 are implemented. The ASIC 410 is connected to an integrated LED array 420, e.g., to terminal and intermediate nodes of a serially-connected string of LEDs implemented in the LED array 420. These circuits may provide functionality along the lines discussed above with reference to FIGS. 1 and 2. The ASIC 410 and LED array 420 may be mounted on a common microelectronic substrate 401.

According to additional embodiments, an LED array such as those discussed above with reference to FIGS. 3 and 4, may have characteristics tailored for use with switching circuits such as those described with reference to FIGS. 1 and 2. For example, when using an array of LEDs with a power-factor correcting switching circuitry along the lines of FIGS. 1 and 2, the number and/or size of the LEDs can be matched with the characteristics of the switching circuitry to achieve a desired current density in the LEDs.

According to further aspects, an LED array for use with such switching circuitry may include circuitry that allows the interconnections of LEDs in the array to be varied. For example, the LED array may include switching circuitry that allows LEDs to be selectively paralleled such that, for example, the number of LEDs driven in parallel increases as the current provided by the switching circuit increases. In this manner, for example, a desired current density in the LEDs may be maintained as dimming varies.

Interconnections among LEDs in the array may also be configurable using components, such as switches or fuses, that may be used to provide different terminal characteristics for different driving voltages. For example, blowing a first set of fuses may provide a 230V string, blowing a second set of fuses may provide a 120V string, and blowing a third set of fuses may provide a 100V string. In still further embodiments, when used with a power factor correcting switching circuit along the lines of that shown in FIG. 2, certain LEDs in a string tend to be “on” for a greater duration than others. For example, referring to FIG. 2, the LED D7 first turned “on” during the cycle of the rectified output of the rectifier circuit 210 is energized for a longer period than the LED D14 activated towards the peak of the rectified waveform. An LED array may be configured to allow periodic reconfiguration of the string of LEDs, such that the LEDs that are “on” for a greater portion of a cycle of a periodic input voltage (e.g., an AC voltage from an AC source) are rotated at a rate that provides equalization of the duty each LED experiences over an extended period of operation.

FIGS. 5A and 5B illustrate a lighting apparatus in the form of a module 500 with yet another alternative configuration. The module 500 includes an array of LEDs 512 disposed on a surface of a first substrate 510. The LED array substrate 510 is mounted on a second substrate 520 that serves as a submount in a flip-chip arrangement, such that the solder bumps 515 on a surface of the LED array substrate 510 opposite the LEDs 512 contact an upper surface of the submount substrate 520. LED switching circuitry, such as that described with reference to FIG. 2 above, may be disposed on and/or in the submount substrate 510. The LED drive circuitry is connected to the LEDs 512 in the LED array substrate 510 using the solder bumps 515, and the submount substrate 520 may be connected to an AC power supply via bonding wires 525 to contacts on an underlying PCB 530. The LEDs 512 in the LED substrate 510 may be internally connected to provide one or more strings of LEDs along the lines discussed above with reference to FIGS. 1 and 2. The LED substrate 510 may be directly disposed on and/or in the submount substrate 510 and/or intervening structures may be present, such as a heat transfer layer formed of graphene or graphite and/or a reflective layer that is used to produce directionality in the LED output.

According to some embodiments, the submount substrate 510 may comprise a silicon substrate. In some embodiments, the submount substrate 520 may be formed of other materials, such as gallium nitride (GaN), silicon carbide (SiC) and/or graphene. The submount substrate 510 may have features therein that enhance thermal transfer from the LED array substrate 520, such as thermal vias and/or a plated-on thermal enhancer to improve thermal conduction to an underlying thermal management structure, such as a heat sink or a metal core in the underlying PCB 530.

As illustrated in FIG. 6, an integrated lighting module having the form factors illustrated in FIGS. 3-5 or any of a variety of other form factors, may be mounted within a lighting apparatus, such as a lamp having a standardized AC power connector. For example, as shown in FIG. 6, a bulb-type lamp device may include a housing 610 with a translucent dome 620. The integrated lighting module may be applicable to any form factor or shape of a lamp device, including, but not limited to bulb-type and tube-type devices. An integrated lighting module 500 (or multiple ones of such modules) along the lines illustrated in FIGS. 5A and 5B may be mounted within the housing 610, with a varying-voltage power (e.g., AC power) provided to the assembly via, for example, a conventional Edison screw-type lamp base 630. In this manner, the lighting assembly 500 may functionally replace the filament of a conventional lamp by providing an integrated assembly that simply requires an AC power input, which may be a normal AC power input or a phase cut AC power input from a dimming device. It will be appreciated that other components, such as thermal structures for conducting heat away from the assembly 500, may also be included in the device 600. It will be understood that various types of different AC power connections may be used, including, but not limited to, screw bases and pin and socket connections. Although the connection between the lamp base 630 and the lighting module 500 may be wired, it will be appreciated that other arrangements may be used. For example, in some lighting apparatus, a lighting module, such as the module 500, may include an integrated AC connector configured to directly mate with a lighting socket or other AC connector. For example, a lighting module along the lines discussed above may include an integral set of contacts or other connector structure that is configured to mate with a matching AC power connector.

According to further embodiments, a lighting apparatus may utilize switching circuit that can dynamically change serial and parallel connections among LEDs responsive to a varying voltage waveform, such as an AC power input. FIG. 7 illustrates a lighting apparatus 700 including a rectifier circuit 110, a voltage controlled current control (VCCC) circuit 120 and a switching circuit 730 comprising a plurality of switches S1, S2, . . . , S11 that, under control of a switch control circuit 732, variably interconnect a plurality of LEDs D1, D2, D3, D4. Along lines described above, the LEDs D1, D2, D3, D4, the rectifier 710, the VCCC 720 and/or the switching circuit 730 may be integrated on a common substrate. In some embodiments, the LEDs D1, D2, D3, D4 may be implemented on a one substrate, and the other control circuitry (e.g., the rectifier 710, VCCC 720 and switching circuit 730) may be implemented in a submount substrate configured to interface with the LED substrate. It will be appreciated that other arrangements may be used, such as ones in which the LEDs D1, D2, D3, D4 and portions of the switching circuit 730, such as the switches S1, S2, . . . , S11, are combined on a single substrate.

The switch control circuit 732 may be configured to vary parallel and serial interconnections among the LEDs D1, D2, D3, D4 responsive to a varying voltage. For example, if the input to the rectifier 710 is an AC voltage, the output of the rectifier 710 may be a full-wave rectified voltage. Referring to FIGS. 8-10, as the rectified voltage increases in magnitude, the switch control circuit 732 may implement a sequence in which the switches S1, S2, . . . , S11 are selectively operated to change from a first state in which the LEDs D1, D2, D3, D4 are connected in parallel (FIG. 8), to a second state in which series-connected pairs of the LEDs D1, D2, D3, D4 are connected in parallel (FIG. 9) and to a third state in which the LEDs D1, D2, D3, D4 are connected in series (FIG. 10). It will be appreciated that this sequence may be reversed as the magnitude of the rectified voltage decreases. In this manner, the number of conducting LEDs may be maintained substantially constant as the rectified voltage changes. It will be appreciated that the sequence of circuit configurations illustrated in FIGS. 8-10 are provided for purposes of illustration, and that other sequences may be used.

In the drawings and specification, there have been disclosed typical preferred embodiments of the inventive subject matter and, although specific terms are employed, they are used in a generic and descriptive sense only and not for purposes of limitation, the scope of the inventive subject matter being set forth in the following claims. 

What is claimed is:
 1. A lighting apparatus comprising: a first substrate comprising a switching circuit; and a second substrate mechanically attached to the first substrate and comprising at least two light emitting diodes (LEDs) interfaced with the switching circuit, the switching circuit being configured to vary interconnections among the at least two LEDs responsive to a varying voltage produced by a power source to provide a plurality of different parallel and serial arrangements of the at least two LEDs including an arrangement wherein at least two parallel interconnected subsets of the at least two LEDs are connected in series with each other.
 2. The apparatus of claim 1, wherein the at least two LEDs have first and second nodes coupled to the power source and at least one third node coupled to the at least one terminal of the switching circuit.
 3. The apparatus of claim 2, wherein the switching circuit is configured to selectively bypass at least one LED of the at least two LEDs responsive to the varying voltage produced by the power source.
 4. The apparatus of claim 3, wherein the varying voltage comprises an AC voltage.
 5. The apparatus of claim 2, wherein the at least two LEDs comprises a string of serially-connected LEDs, wherein the first and second nodes comprise respective first and second terminal nodes of the string and wherein the at least one third node comprises an intermediate node of the string.
 6. The apparatus of claim 2, wherein the first substrate further comprises a first port coupled to the power source and a second port coupled to the first port and to the first and second nodes of the at least two LEDs.
 7. The apparatus of claim 6, wherein the first substrate further comprises a current control circuit coupled to the first and second ports.
 8. The apparatus of claim 1, wherein the first and second substrates are planar and attached in a stacked arrangement.
 9. The apparatus of claim 1, wherein at least one of the first and second substrates is disposed in a housing.
 10. A device comprising: a control circuit substrate configured to be mechanically attached to an LED substrate and comprising a switching circuit and a plurality of contacts coupled to the switching circuit and configured to be electrically coupled to corresponding contacts of the LED substrate, wherein the switching circuit is configured to vary interconnections among at least two LEDs on the LED substrate responsive to a varying voltage produced by a power source to provide a plurality of different parallel and serial arrangements of the at least two LEDs including an arrangement wherein at least two parallel interconnected subsets of the at least two LEDs are connected in series with each other.
 11. The device of claim 10, wherein the control circuit substrate comprises a first port configured to be coupled to the power source, a second port coupled to the first port, and wherein the plurality of contacts including at least one contact coupled to the second port and at least one contact coupled to the at least one terminal of the switching circuit.
 12. The device of claim 11, wherein the switching circuit is configured to selectively bypass at least one LED of the at least two LEDs on the LED substrate responsive to the varying voltage at the first port.
 13. The device of claim 12, wherein the varying voltage comprises an AC voltage.
 14. The device of claim 11, wherein the control circuit substrate further comprises a current control circuit coupled to the first port and the second port.
 15. The apparatus of claim 14, wherein the second planar substrate further comprises a first port configured to be coupled to the power source and a second port coupled to the first port and to at least one node of the at least two LEDs.
 16. The apparatus of claim 15, wherein the second planar substrate further comprises a current control circuit coupled between the first and second ports.
 17. The device of claim 11, wherein the control circuit substrate is planar and configured to be mechanically attached to the LED substrate in a stacked arrangement.
 18. The apparatus of claim 17, further comprising a housing containing at least one of the first and second planar substrates.
 19. An apparatus comprising; a first planar substrate comprising at least two LEDs; and a second planar second substrate mechanically attached to the first planar substrate in a stacked relationship and comprising a switching circuit electrically coupled to the at least two LEDs and configured to vary interconnections among the at least two LEDs responsive to a varying voltage produced by a power source to provide a plurality of different parallel and serial arrangements of the at least two LEDs including an arrangement wherein at least two parallel interconnected subsets of the at least two LEDs are connected in series with each other.
 20. The apparatus of claim 19, wherein the switching circuit is configured to selectively bypass at least one LED of the at least two LEDs responsive to the varying voltage. 